Cooperative Carbon Dioxide Capture in Diamine-Appended Magnesium–Olsalazine Frameworks

Diamine-appended Mg2(dobpdc) (dobpdc4– = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) metal–organic frameworks have emerged as promising candidates for carbon capture owing to their exceptional CO2 selectivities, high separation capacities, and step-shaped adsorption profiles, which arise from a unique cooperative adsorption mechanism resulting in the formation of ammonium carbamate chains. Materials appended with primary,secondary-diamines featuring bulky substituents, in particular, exhibit excellent stabilities and CO2 adsorption properties. However, these frameworks display double-step adsorption behavior arising from steric repulsion between ammonium carbamates, which ultimately results in increased regeneration energies. Herein, we report frameworks of the type diamine–Mg2(olz) (olz4– = (E)-5,5′-(diazene-1,2-diyl)bis(2-oxidobenzoate)) that feature diverse diamines with bulky substituents and display desirable single-step CO2 adsorption across a wide range of pressures and temperatures. Analysis of CO2 adsorption data reveals that the basicity of the pore-dwelling amine—in addition to its steric bulk—is an important factor influencing adsorption step pressure; furthermore, the amine steric bulk is found to be inversely correlated with the degree of cooperativity in CO2 uptake. One material, ee-2–Mg2(olz) (ee-2 = N,N-diethylethylenediamine), adsorbs >90% of the CO2 from a simulated coal flue stream and exhibits exceptional thermal and oxidative stability over the course of extensive adsorption/desorption cycling, placing it among top-performing adsorbents to date for CO2 capture from a coal flue gas. Spectroscopic characterization and van der Waals-corrected density functional theory calculations indicate that diamine–Mg2(olz) materials capture CO2 via the formation of ammonium carbamate chains. These results point more broadly to the opportunity for fundamentally advancing materials in this class through judicious design.


Thermogravimetric Decomposition Data
Dry N2 decomposition profiles of Mg 2 (olz). A ramp rate of 2 °C/min was used.  Figure S4.
Scanning electron microscopy images of Mg2(olz) and ee-2-Mg2(olz) crystallites. Images were taken using a Hitachi S-5000 SEM at the Electron Microscope Laboratory at the University of California, Berkeley. Samples were dispersed in methanol and then drop casted onto silicon chips. To dissipate charge, samples were sputter coated with approximately 3 nm of gold (Tousimis). Scale bars: 1 µm. Figure S5.

Thermogravimetric Decomposition Data
Dry N2 decomposition profiles for diamine-Mg 2 (olz) analogues. The thermogravimetric decomposition traces are marked in black while the derivative decomposition traces (dTG) are marked in blue. The lower temperature mass loss (<80 °C) corresponds to weakly absorbed species (e.g., CO2, H2O, toluene, hexane), the middle temperature mass loss (~100 °C) corresponds to excess diamines in the pore, and the higher temperature mass loss (>200 °C) corresponds to the metal-bound diamines. A ramp rate of 2 °C/min was used.  Table S3. Diamines used in this work to prepare diamine-Mg2(olz) frameworks and the abbreviation used for each. The primary amine in each case (and the least sterically hindered primary amine in dmen) was presumed to bind to the framework Mg 2+ sites as discussed in the main text. To evaluate any correlation between the basicity of the pore-dwelling amine and the step pressure/temperature for CO2 uptake, a representative structure was first generated by replacing the metal-bound primary amine in each case with a proton, see the structures in column 4 below. Experimental pKa values for the corresponding monoammonium cations are reported when available (in parentheses), 1

Hill Coefficients
* The Hill equation is as follows 3 where q is the fraction of the bound to total receptors, [L] is the total ligand concentration, Kd is the dissociation constant, n is the Hill coefficient, and K0.5 is the ligand concentration at which half of the receptors are bound and equivalent to the nth root of Kd. In our analysis, we used the following equation: Here, q CO 2 is defined as the fraction of bound CO2 to the total capacity at the top of the adsorption step (in order to exclude physisorbed CO2, which is reflected in the gradual CO2 uptake following the adsorption step); P is the CO2 pressure; P50% is the pressure when 50% of the diamines in the material have bound CO2 (based on the diamine loadings determined via 1 H NMR spectroscopy); and n is the Hill coefficient. The fits were performed using MATLAB. 4

Calculation of the Approximate Regeneration Energy of ee-2-Mg 2 (olz)
Regeneration energies were calculated using the following approach: Dhads is the differential enthalpy of adsorption in kJ/molCO2; Dhads of ee-2-Mg2(olz) is 69.9 kJ/mol.

Breakthrough Experiment Details
As-synthesized ee-2-Mg2(olz) powder was filtered from the amine/toluene solution (see the Experimental Section) and then compressed into a tablet using mechanical press. The tablet was then broken into pellets using 45 mesh sieves and collected via 25 mesh sieves so that the pellet sizes were between 350 and 700 µm in diameter. These pellets were then activated on a Schlenk line with heating at 130 °C for 1 h under a continuous N2 purge. The breakthrough column, comprising a 6" section of 1/4" stainless steel pipe (i.d. 0.18") with Swagelok fittings on each side, was packed with 0.58 g of activated ee-2-Mg2(olz) pellets, and glass wool was added to each end of the column to secure the pellets.
The CO2 capture performance of this material under multicomponent conditions was then measured via a custom-built breakthrough apparatus, containing a Parker-Porter mass flow controller, copper and stainless steel tubing (1/8") with Swagelok fittings and valves to control the gas flow, and an Agilent ADM200 Universal Flow Meter measuring the outlet gas flow rate. The outlet gas concentration was measured at 1 min intervals via an SRI Instruments 8610C gas chromatograph equipped with a Hayesep D column and TCD detector. The instrument was calibrated using a series of premixed standard tanks of varying concentrations of CO2 (5, 10, 15, 20, 30, and 50%) in N2 purchased from Praxair, as well as pure CO2 and N2. Peak integration and analysis were calculated using PeakSimple software.
The column was first activated at 130 °C under 10 sccm of He flow for 1 h, then cooled to 40 °C for measurement. The breakthrough measurements were then carried out using a 15% CO2 in N2 premixed cylinder from Praxair at a flow rate of 10 sccm and atmospheric pressure. The material was reactivated at 85 °C for 1 h under 10 sccm of He flow between each measurement. The CO2 and N2 uptake capacities ( N , in mmol/g) were calculated via the following formula: where N,* and N are the inlet and outlet mass flowrates of species in sccm, is the sample mass in g, is the time since the start of the breakthrough measurement in minutes, N is the mole fraction in the inlet gas stream, is the volume of the adsorbent column in standard cc, and are the pressure and temperature of the system, and is the universal gas constant. The void fraction of the column, , is made up of the empty space between particles due to their packing and is equal to approximately 0.4 for spherical particles of this size. 5 The error in reported capacity was calculated by propagating the standard deviation of the measured outlet flowrate, the most variable measured value, through the capacity calculation.
For humid breakthrough experiments, a fritted water bubbler was added upstream of the adsorbent column, providing an H2O concentration of ~2% assuming saturation at room temperature (20 °C). The breakthrough column was pre-saturated with H2O using 10 sccm of humid He overnight prior S27 to each experiment. Pre-saturation of the material was confirmed by a column of indicating Drierite downstream of the adsorption column, which was removed after saturation and before beginning the breakthrough measurement. Furthermore, the bubbler was also pre-saturated with CO2 by closing off the adsorbent column and flowing the 15% CO2 in N2 stream through the bubbler and then through the bypass directly to the GC until a stable concentration of 15% CO2 was recorded, indicating that CO2 was no longer dissolving into the bubbler water, a process which typically took 30-60 min.
Additional Solid-state Magic Angle Spinning 13 C NMR Spectra and Details

Van der Waals-Corrected DFT Calculations
To elucidate the effect of framework size on CO2 adsorption, we performed first-principles density functional theory (DFT) calculations using a plane-wave basis and projector augmented-wave (PAW) 6-7 pseudopotentials with the Vienna ab-initio Simulation Package (VASP) code. [8][9][10][11] To include the effect of the van der Waals (vdW) dispersive interactions on binding energies, we performed structural relaxations with vdW dispersion-corrected functionals (vdW-DF2) 12 as implemented in VASP. For all calculations, we used (i) a Γ-point sampling of the Brillouin zone and (ii) a 600-eV plane-wave cutoff energy. We explicitly treated two valence electrons for Mg (3s 2 ), six for O (2s 2 2p 4 ), five for N (2s 2 2p 3 ), four for C (2s 2 2p 2 ), and one for H(1s 1 ). All structural relaxations were performed with a Gaussian smearing of 0.05 eV. 13 The ions were relaxed until the Hellmann-Feynman forces are less than 0.02 eVÅ −1 .